main.dart

import 'dart:convert';

void main() {
  
  List decoded = json.decode("""[
    {
        "date": "2018-08-23 08:00:00",
        "end": "2018-08-23 08:25:00",
        "info": "Advisement",
        "isclass": "1",
        "period": "-4"
    },
    {
        "date": "2018-08-23 08:31:00",
        "end": "2018-08-23 09:12:00",
        "info": "nan",
        "isclass": "1",
        "period": "1"
    },
    {
        "date": "2018-08-23 09:18:00",
        "end": "2018-08-23 09:59:00",
        "info": "nan",
        "isclass": "1",
        "period": "2"
    }]""");
  
  var now = new DateTime.now();
  
  print(decoded.where((i) => DateTime.parse(i['end']).isAfter(now)).toList());
  
}